Charge-transfer complexes of dibenzofuran-formaldehyde or dibenzothiophene-formaldehyde resins as photoconductive materials

ABSTRACT

Certain formaldehyde resins are disclosed which form complexes with Lewis acids such as trinitrofluorenone to produce photoconductive compositions and elements of high speed and utility for electrostatic image formation.

Elite States Contois et al.

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[151 3,655,378 [4 Apr.11,l972

[54] CHARGE-TRANSFER COMPLEXES OF DIBENZOFURAN-FORMALDEHYDE ORDIBENZOTHIOPHENE-FO- I RMALDEHYDE RESIN S AS PHOTO- CONDUCTIVE MATERIALS[72] Inventors: Lawrence E. Contois, Webster; Stewart H.

Merrill, Rochester, both of, NY; George S. Gran, Hightstown, NJ.

[73] Assignee: Eastman Kodak Company, Rochester,

[22] Filed: Mar.1,197l

[21] Appl.No 119,956

[52] user ..96/1.5, 96/1 PC, 252/501,

260/67 FP, 260/67 A 51 in. CI. ..G03g 5/06 [58] Field ofSearch ..260/67FP, 67 A; 96/1.5, 1;

Primary Examiner-George F. Lesmes Assistant Examiner-John C. Cooper, IllAn0rneyR. W. Hampton, P. R. Holmes and T. Hiatt [57] ABSTRACT Certainformaldehyde resins are disclosed which form complexes with Lewis acidssuch as trinitrofluorenone to produce photoconductive compositions andelements of high speed and utility for electrostatic image formation.

10 Claims, No Drawings CHARGE-TRANSF ER COMPLEXES OF DIBENZOFURAN-FORMALDEHYDE OR DIBENZOTHIOPHENE- FORMALDEHYDE RESINS AS PHOTOCONDUCTIVEMATERIALS This invention relates to electrophotography, and inparticular to photoconductive compositions and elements.

The process of xerography, as disclosed by Carlson in US. Pat. No.2,297,691, employs an electrophotographic element comprising a supportmaterial bearing a coating of a normally insulating material whoseelectrical resistance varies with the amount of incident electromagneticradiation it receives during an imagewise exposure. The element,commonly termed a photoconductive element, is first given a uniformsurface charge, generally in the dark after a suitable period of darkadaptation. It is then exposed to a pattern of actinic radiation whichhas the effect of differentially reducing the potential of this surfacecharge in accordance with the relative energy contained in various partsof the radiation pattern. The differential surface charge orelectrostatic latent image remaining on the electrophotographic elementis then made visible by contacting the surface with a suitableelectroscopic marking material. Such marking material or toner, whethercontained in an insulating liquid or on a dry carrier, can be depositedon the exposed surface in accordance with either the charge pattern ordischarge pattern as desired. Deposited marking material can then beeither permanently fixed to the surface of the sensitive element byknown means such as heat, pressure, solvent vapor, or the like, ortransferred to a second element to which it can similarly be fixed.Likewise, the electrostatic charge pattern can be transferred to asecond element and developed there.

Various photoconductive insulating materials have been employed in themanufacture of electrophotographic elements. For example, vapors ofselenium and vapors of selenium alloys deposited on a suitable supportand particles of photoconductive zinc oxide held in a resinous,film-forming binder have found wide application in the present-daydocument copying applications.

Since the introduction of electrophotography, a great many organiccompounds have also been screened for their photoconductive properties.As a result, a very large number of organic compounds have been known topossess some degree of photoconductivity. Many organic compounds haverevealed a useful level of photoconduction and have been incorporatedinto photoconductive compositions. Typical of these organicphotoconductors are the triphenylamines and the triarylmethane leucobases. Optically clear photoconductor-containing elements havingdesirable electrophotographic properties can be especially useful inelectrophotography. Such electrophotographic elements can be exposedthrough a transparent base if desired, thereby providing unusualflexibility in equipment design. Such compositions, when coated as afilm or layer on a suitable support, also yield an element which isreusable; that is, it can be used to form subsequent images afterresidual toner from prior images has been removed by transfer and/orcleaning.

It is, therefore, an object of this invention to provide a novel classof photoconductors having high photosensitivity when electricallycharged.

it is another object to provide novel photoconductor-containingcompositions which exhibit high electrical speeds.

It is a further object of the invention to provide an improved processutilizing the novel photoconductors described herein.

These and other objects are accomplished by employing certaindibenzofuran-formaldehyde resins or dibenzothiaphene-formaldehyde resinswhich form a complex with certain Lewis acids such as2,4,7-trinitro-9-fluorenone, 2,4,5,7-tetranitro-9-fluorenone,2,6-dichloro-p-benzoquinone, 2,5-dinitro-9-fluorenone, l,5-dichloro-2,4-dinitrobenzene, 2,5-dichloro-p-benzoquinone, tetrachloroquinone,2-chloro-3,S-dinitropyridine, and2,4,5,7,9-pentanitro-indeno[2,l-alfluoren-l l,l2-dione, 2,5-diphenyl-p-benzoquinone, 2,3-dichloro-1,4-naphthoquinone,

9-dicyanomethylene-2,4,7-trinitrofluorene, that are accompanied byspectral shifts. The photoconductive efficiency of said combinations isquite high and the observed photoconductivity can be further enhanced byemploying additional sensitizers.

The compounds which form the complex of the present invention includeresins which are formed by the condensation of dibenzothiophene withformaldehyde and and dibenzofuran with formaldehyde and which compriseunits having the formula wherein X is an oxygen atom or a sulfur atom,and an aforementioned Lewis acid such as 2,4,7-trinitro-9-fluorenone(TNF). The combination of such compounds asdibenzothiophene-formaldehyde resin with TNF exhibits greatly increasedspeed and efficiency over the combination of dibenzothiophene itselfcomplexed with TNF.

According to this invention, it has been found that the photoconductorcomplexes described herein have enhanced speed over those photoconductorcomplexes described in the prior art. In particular, substantialincreases in speeds are obtained as compared to speeds attainable withmany other closely related compounds. These increases in speed areobserved when the coating accepts a suitable potential (e.g., 500-600volts) and the relative speed of the coating is determined on the basisof the reciprocal of the exposure required to reduce the potential ofthe surface charge by volts (shoulder speed) or to 100 volts (toespeed). The terms shoulder speed and toe speed are terms known in thephotographic art with reference to H and D curves. As used herein, suchterms refer to corresponding curves resulting from exposure plottedagainst voltage. The reduction of the surface potential to 100 volts orbelow is significant in that it represents a requirement for suitablebroad area development of an electrostatic image. The relative speed at100 volts is a measure of the ability to produce and hence to develop orotherwise utilize the electrostatic image. When many conventionalphotoconductors are used, the surface potential frequently does not dropto or below 100 volts and therefore no speed can be assigned to such acomposition. When most photoconductors are used in'photoconductivecompositions, the surface potentials of such resultant compositionsusually drop below 100 volts and thus, a definite speed can beascertained. However, these speeds are improved when the photoconductorscomplexes of this invention are employed. The preparation of the complexis performed by simply mixing the constituents together when preparing adope for the preparation of coatings.

Electrophotographic elements of the invention can be prepared to containthe photoconducting complexes of the present invention in theconventional manner, i.e., by blending a dispersion or solution of aphotoconductive complex or the constituent compounds together with abinder, when necessary or desirable, and coating or forming aself-supporting layer with the materials. Likewise, otherphotoconductors known in the art such as those described in Light,British Pat. No. 1,153,506 dated May 29, 1969, can be combined with thepresent photoconductors. In addition, supplemental materials useful forchanging the spectral sensitivity or electrophotosensitivity of theelement can be added to the composition of the element when it isdesirable to produce the characteristic effect of such materials.

The photoconductive layers of the invention can also be sensitized bythe addition of effective amounts of sensitizing compounds to exhibitimproved electrophotosensitivity. Sensitizing compounds useful with thephotoconductive compounds of the present invention can be selected froma wide variety of materials, including such materials as pyrylium dyesalts including thiapyrylium dye salts and selenapyrylium dye saltsdisclosed in VanAllan et al., U.S. Pat. No. 3,250,615 issued May 10,1966; hexaphenyl-p-rosaniline; fluorenes, such as 7,1 Z-dioxo-l3,dibenzo(a,h)fluorene, 5, lO-dioxo-4a,l ldiazobenzo( b )fluorene,3,13-dioxo-7-oxadibenzo(b,g)fiuorene, and the like; aggregate-typesensitizers of the type described in Light, British Pat. No. 1,153,506,dated May 29, 1969; aromatic nitro compounds of the kinds described inMinsk U.S. Pat. No. 2,610,120 issued Sept. 9, 1952; anthrones like thosedisclosed in Zvanut U.S. Pat. No. 2,670,284 issued Feb. 23, 1954;quinones, Minsk U.S. Pat. No. 2,670,286 issued Feb. 23,1954;benzophenones, Minsk U.S. Pat. No. 2,670,287 issued Feb. 23, 1954;thiazoles Robertson U.S. Pat. No. 2,732,301 issued Jan. 24, 1956;mineral acids; carboxylic acids, such as maleic acid, dichloroaceticacid, trichloroacetic acid and salicylic acid; sulfonic and phosphoricacids; 2,7-dimethyl-9-(p-tolyl)-lO-(ptolyl)-9-acridinol;N-(p-diphenylaminobenzal)-N'-(3-rnethyl- 2-benzothiazal)hydrazine;p-diphenylaminocinnamic acid; 4,4,4"-trimethoxyphenylamine; and variousdyes, such as cyanine (including carbocyanine and polycarbocyanine.merocyanine, diarylmethane, thiazine, azine, oxaxine, xanthene,phthalein, acridine, azo, anthraquinone dyes and the like; mixturesthereof.

When a sensitizing compound is employed with an organic photoconductorcomplex to form a sensitized electrophotographic element, it is thenormal practice to mix a suitable amount of the sensitizing compoundwith the coating composition so that, after thorough mixing, thesensitizing compound is uniformly distributed in the coated layer.

Other methods of incorporating the sensitizer or the effect of thesensitizer may, however, be employed consistent with the practice ofthis invention. In preparing the photoconductive layers, no sensitizingcompound is required to give photoconductivity in the layers whichcontain the photoconducting substances, therefore, no sensitizer isrequired in a particular photoconductive layer. However, sincerelatively minor amounts of sensitizing compound give substantialimprovement in speed in such layers, the sensitizer is preferred. Asseen from the previous listing both spectral and chemical sensitizerscan be used in this invention, either alone or in combination. Theamount of sensitizer that can be added to a photoconductive layer togive efiective increases in speed can vary widely. The optimumconcentration in any given case will vary with the specificphotoconductor and sensitizing compound used. In general, substantialspeed gains can be obtained where an appropriate sensitizer is added ina concentration range from about 0.000l to about 30 percent by weightbased on the weight of the film-forming coating composition. Normally, asensitizer is added to the coating composition in an amount by weightfrom about 0.005 to about 5.0 percent by weight of the total coatingcomposition.

While the Lewis acid and dibenzofuran-formaldehyde resin ordibenzothiophene-formaldehyde resin may be used together withoutadditional binder materials, it is sometimes desirable for binders to beemployed. Preferred binders include materials which are film-forming,hydrophobic polymeric binders having fairly high dielectric strengthwhich are good electrically insulating film-forming vehicles.

Typical of these materials are:

1. Natural resins including gelatin, cellulose ester derivatives such asalkyl esters of carboxylated cellulose, hydroxy ethyl cellulose, carboxymethyl cellulose, carboxy methyl hydroxy ethyl cellulose, etc.;

ll. Vinyl resins including a. polyvinyl esters such as a vinyl acetateresin, a copolymer of vinyl acetate and crotonic acid, a copolymer ofvinyl acetate with an ester of vinyl alcohol and a higher aliphaticcarboxylic acid such as lauric acid or stearic acid, polyvinyl stearate,a copolymer of vinyl acetate and maleic acid, a poly(vinylhaloarylate)such as poly(vinylm-bromo-benzoate-co-vinyl acetate), a terpolymer ofvinyl butyral with vinyl alcohol and vinyl acetate, etc.;

b. vinyl chloride and vinylidene chloride polymers such as apoly(vinylchloride), a copolymer of vinyl chloride and vinyl isobutylether, a copolymer of vinylidene chloride and acrylonitrile, aterpolymer of vinyl chloride, vinyl acetate and vinyl alcohol,poly(vinylidene chloride) a terpolymer of vinyl chloride, vinyl acetateand maleic anhydride, a copolymer of vinyl chloride and vinyl acetate,etc.;

0. styrene polymers such as polystyrene, a nitratecl polystyrene, acopolymer of styrene and monoisobutyl maleate, a copolymer of styrenewith methacrylic acid, a copolymer of styrene and butadiene, a copolymerof dirnethyl itaconate and styrene, polymethylstyrene, etc.;

d. methacrylic acid ester polymers such as a poly(akylmethacrylate),etc.;

e. polyolefins such as chlorinated polyethylene, chlorinatedpolypropylene, poly(isobutylene), etc.;

f. poly(vinyl acetals) such as poly(vinyl butyral), etc.; and

g. poly( vinyl alcohol);

lll. Polycondensates including a. a polyester of 1,3-disulfobenzene and2,2-bis(4-hydroxyphenyl)propane;

b. a polyester of diphenyl-p,p'-disulphonic acid and 2,2-

bis(4-hydroxyphenyl)propane:

c. a polyester of 4,4:-dicarboxyphenyl ether and 2,2-bis(4-hydroxyphenyl)propane;

d. a polyester of 2,2-bis(4hydroxyphenyl)propane and fumaric acid;

e. polyester of pentaerythritol and phthalic acid;

f. resinous terpene polybasic acid;

g. a polyester of phosphoric acid and hydroquinone;

h. polyphosphites;

i. polyester of neopentylglycol and isophthalic acid;

j. polycarbonates including polythiocarbonates such as the polycarbonateof 2,2-bis( 4-hydroxyphenyl)propane;

k. polyester of isophthalic acid,2,2-bis{4-(fl-hydroxyethoxy)phenyl1propane and ethylene glycol;

l. polyester ofterephthalic acid, 2,2-bis[4-(fl-hydroxyethoxy)phenyl]propane and ethylene glycol;

m. polyester of ethylene glycol, neopentyl, glycol,

terephthalic acid and isophthalic acid;

n. polyamides;

o. ketone resins; and

p. phenolforrnaldehyde resins;

IV. Silicone resins;

V. Alkyd resins including styrene-alkyd resins, siliconealkyd resins,soya-alkyd resins, etc.;

Vl. Paraffin; and

VII. Mineral waxes.

Solvents useful for preparing coating compositions with thephotoconductors of the present invention can include a wide variety oforganic solvents for the components of the coating composition.

Typical solvents include:

1. Aromatic hydrocarbons, such as benzene, etc., including substitutedaromatic hydrocarbons, such as toluene, xylene, mesitylene, etc.;

2. Ketones, such as acetone, 2-butanone, etc.;

3. Halogenated aliphatic hydrocarbons, such as methylene chloride,chloroform, ethylene chloride, etc.;

4. Ethers including cyclic ethers such as tetrahydrofuran,

dioxane; and

5. Mixtures of the above.

In preparing the photoconductive coating compositions of the presentinvention useful results are obtained where the photoconductive complexis present in an amount equal to at least about 0. 1 weight percent ofthe coating composition. The upper limit in the amount ofphotoconductive material present can be widely varied to at leastpercent by weight in accordance with usual practice.

Coating thicknesses of the photoconductive layer on a support can varywidely. Normally, a wet coating thickness in the range of about 0.001inch to about 0.01 inch is useful in the practice of the invention. Apreferred range of coating thickness is from about 0.002 inch to about0.006 inch before drying although such thicknesses can vary widelydepending on the particular application desired for theelectrophotographic element.

Suitable supporting materials for the photoconductive layers of thepresent invention can include any of the electrically conductingsupports, for example, various conducting papers; aluminum-paperlaminates; metal foils, such as aluminum foil, zinc foil, etc.; metalplates such as aluminum, copper, zinc, brass, and galvanized plates;vapor deposited metal layers such as silver, nickel or aluminum onconventional film supports, such as cellulose acetate, poly(ethyleneterephthalate), polystyrene and the like conducting supports.

An especially useful conducting support can be prepared by coating atransparent film support material such as poly- (ethylene terephthalate)with a layer containing a semiconductor dispersed in a resin. A suitableconducting coating can be prepared from the sodium salt of acarboxy-ester lactone of a maleic anhydride-vinyl acetate copolymer,cuprous iodide and the like. Such conducting layers and methods fortheir optimum preparation and use are disclosed in U.S. Pat. No.3,007,901 by Minsk issued Nov. 7, 1961; U.S. Pat. No. 3,262,807 bySterman issued July 26, 1966, and U.S. at. No. 3,245,833 by Trevoyissued Apr. 12, 1966.

The compositions of the present invention can be employed inphotoconductive elements useful in any of the well knownelectrophotographic processes which require photoconductive layers. Onesuch process is the xerographic process. In a process of this type, anelectrophotographic element held in the dark, is given a blanketpositive or negative electrostatic charge as desired by placing it undera corona discharge to give a uniform charge to the surface of thephotoconductive layer. This charge is retained by the layer owing to thesubstantial dark insulating property of the layer, i.e., the lowconductivity of the layer in the dark. The electrostatic charge formedon the surface of the photoconductive layer is then selectivelydissipated from the surface of the layer by imagewise exposure to lightby means of a conventional exposure operation such as for example, by acontact-printing technique, or by lens projection of an image, or reflexor bireflex techniques and the like, to thereby form a latentelectrostatic image in the photoconductive layer. Exposing the surfacein this manner forms a pattern of electrostatic charge by virtue of thefact that light energy striking the photoconductor causes theelectrostatic charge in the light struck areas to be conducted away fromthe surface in proportion to the illuminance on a particular area.

The charge pattern produced by exposure is then developed or transferredto another surface and developed there, i.e., either the charged oruncharged areas rendered visible, by treatment with a medium comprisingelectrostatically responsive particles having optical density. Thedeveloping electrostatically responsive particles can be in the form ofa dust, or powder and generally comprise a toner formed ofa pigment in aresin binder. A preferred method of applying such a toner to anelectrostatic image for solid area development is by the use of amagnetic brush. Methods of forming and using a magnetic brush tonerapplicator are known in the art, e.g.: U.S. Patents No. 2,786,439 byYoung issued Mar. 26, 1957; U.S. Pat. No. 2,786,440 by Giaimo issuedMar. 26, 1957; and U.S. Pat. No. 2,786,441 by Young issued Mar. 26,1957. Liquid development of the latent electrostatic image may also beused. In liquid development the developing particles are carried to theimage-bearing surface in an electrically insulating liquid carrier.Methods of development of this type are widely known and have beendescribed in the patent literature, for example, U.S. Patent No.2,907,674 by Metcalfe et al. issued Oct. 6, 1959. In dry developingprocesses the most widely used method of obtaining 11 permanent recordis achieved by selecting a developing particle which has as one of itscomponents a low-melting resin. Heating the powder image then causes theresin to melt or fuse into or on the element. The powder is, therefore,

caused to adhere permanently to the surface of the photoconductivelayer. In other cases, a transfer of the charge image or powder imageformed on the photoconductive layer can be made to a second support suchas paper which would then become the final print after developing andfusing or fusing respectively. Techniques of the type indicated are wellknown in the art and have been described in the literature such as inRCA Review, vol. 15 (1954) pages 469-484.

The compositions of the present invention can be used inelectrophotographic elements having many structural variations. Forexample, the photoconductive composition can be coated in the form ofsingle layers or multiple layers on a suitable opaque or transparentconducting support. Likewise, the layers can be contiguous or spacedhaving layers of insulating material or other photoconductive materialbetween layers or overcoated or interposed between the photoconductivelayer or sensitizing layer and the conducting layer. It is also possibleto adjust the position of the support and the conducting layer byplacing a photoconductor layer over a support and coating the exposedface of the support or the exposed or overcoated face of thephotoconductor with a conducting layer. Configurations differing fromthose contained in the examples can be useful or even preferred for thesame or different application for the electrophotographic element.

The following examples are included for a further understanding of thisinvention.

EXAMPLE 1 To a boiling solution of 3.0 grams (0.10 mole) ofparaformaldehyde in ml. of formic acid is added 16.8 grams (0.10 mole)of dibenzofuran. After 6 hours of reflux, the liquid is decanted fromthe solid which separates, and the solid is dissolved indichloromethane. The residual formic acid is removed by extraction withaqueous potassium carbonate. Evaporation of the dichloromethane givesthe resinous product, dibenzofuran-formaldehyde. The resinousdibenzothiophene-formaldehyde is prepared in the same manner usingdibenzothiophene as the starting material in place of the dibenzofuran.A first dope (A) is prepared con taining the following components:

puly(4,4'-isopropylidenediphenylene carbonate) 0.75 g.dibenzofuran-formaldehyde resin 0.75 g, 2,4,7-trinitro-9-fluorenone(TNF) 050 g. dichloromethane 11.7 ml.

A second dope (B), similar to the first, is prepared, in which thedibenzofuran-formaldehyde resin is replaced with an equal weight ofdibenzothiophene-formaldehyde resin. A third dope (C), similar to thefirst, is prepared as a control, in which the dibenzofuran-formaldehyderesin is replaced completely with an equal weight of the polycarbonate.A fourth dope (D), similar to the second, is prepared as a control, inwhich the dibenzothiophene-formaldehyde resin is replaced with an equalweight of dibenzothiophene. Each dope is hand coated at a wet thicknessof microns on suitable conductive support such as evaporated nickel on apoly(ethylene terephthalate) film support. The coating block ismaintained at a temperature of about 32 C. In a darkened room, thesurface of each of the photoconductive layers so prepared is charged toa potential of about +600 volts under a corona charger. Each layer inturn is covered with a transparent sheet bearing a pattern of opaque andlight-transmitting areas and exposed to the radiation from anincandescent lamp with an illuminance of about 75 meter-candles for 12seconds. Each of the resulting electrostatic charge patterns isdeveloped by cascading over the surface of each layer a developercomprising negatively charged black thermoplastic toner particles onglass bead carriers. The quality of the images reproduced using thevarious photoconductors described herein is set forth in Table I.

TABLE 1 Element No. Dope Image Quality 1 A Good 2 B Good 3 C (control)Poor 4 D (control) No Image EXAMPLE 2 Each of the electrophotographicelements prepared in Example 1 is charged under a positive or negativecorona source until the surface potential, as measured by anelectrometer probe, reaches about 600 volts. It is then exposed frombehind a stepped density gray scale to a light source using either a3000 K. tungsten source or a xenon arc. The exposure causes reduction ofthe surface potential of the element under each step of the gray scalefrom its initial value, V to some lower value V whose exact valuedepends on the actual amount of exposure received by the area under thestep. The results of the measurements are plotted on a graph of surfacepotential V vs. log exposure for each step. The shoulder speed is thenumerical expression of multiplied by the reciprocal of the exposure inmeter-candle-seconds required to reduce the 600 volt charged surfacepotential by 100 volts. The toe speed is the numerical expression of 10multiplied by the reciprocal of the exposure in meter-candle-secondsrequired to reduce the 600 volt charged surface potential to 100 volts.An apparatus for automatically measuring the potentials and recordingthen is described in Robinson et al. US. Pat. No. 3,449,658, issued June10, 1969. The speeds obtained for the various elements tested are givenin Table II below.

TABLE IL-ELEC'IROPHOTO GRAPHIC SPEEDS Positive Negative Absorptionshoulder/ shoulder/ Element; N 0. Dope extends to toe toe Source 560 nm2,500/100 5, 000/450 Xenon. 590 nm t 5, 700/400 9,000/1,100 Do. 440 nm./6. 3 1, 200/ Do. 540 nm 65/2. 0 200/12 Tungsten. 590 nm 1, 000/14630/32 D0.

The combination of TNF and the resins in dopes A, B and D producecomplexes evidenced by shifts in spectral absorption. Dopes C and D, inwhich the dibenzothiophene-formaldehyde resin is absent, produceelements having lower speeds than those obtained with dopes A and B.Similar results are obtained when the TNF is replaced with an equalweight of 2,4,5,7,9-pentanintroindeno[2,l-alfiuoren-l l,l2-dione and9-dicyanomethylene-2,4,7-trinitrofluorene.

EXAMPLE 3 Coating compositions are made according to the procedure ofExample 1 in which the polycarbonate binder is replaced in its entiretywith an equal weight of the polymer poly(4,4-isopropylidenebisphenyleneoxyethylene co ethylene terephthalate). One ofthe compositions contains no sensitizing dye, and serves as the control.Each of the remaining compositions contains one of the dye sensitizerslisted hereinafter.

I. 3,3-diethyl-9-methylthiacarboxyanine bromide 11.3,3-diethylthiadicarbocyanine bromide 111. 3,3-diethylthiatricarbocyanine iodide IV.6-chloro-l-methyl-1,2,3triphenylimidazo4.5-b-quinoxalino-3'-indolocarbocyanine-p-toluenesulfonate V.6,6-dichloro-l l ,3,3-tetraphenylimidazo 4,5-b-quinoxalinocarbocyaninep-toluenesulfonate Vl. Rhodamine B (Cl. 45,170)

V". 2,6-bis( 4-ethylphenyl)-4-( 4-n-amyloxyphenyl )-thiapyryliumperchlorate Vlll. hexaphenyl-p-rosaniline diformate Elements preparedfrom each of these compositions are tested according to the proceduresgiven in Examples 1 and 2 above. When tested according to the procedureof Example 1, each gives a visible image. When tested according to theprocedure of Example 2, using tungsten exposure, speeds are obtained assummarized in Table Ill below.

Each of the dyes is present in an amount of 1 percent of the totalsolids content, with the exception of Dyes Ill and VII, which arepresent at twice that concentration. Only negative toe speeds aremeasured. It is seen that the sensitizing dyes generally increase thespeed or absorption range of the photoconductor-containing compositionsand elements of the invention.

EXAMPLE 4 Coating compositions are made according to the controlcomposition of Example 3, that is, containing no sensitizing dye. Toeach of these compositions is added an amount of one of chemicalsensitizers E, F, G, H and J as indicated hereinafter.

F N-( p-diphenylaminobenzal)-N-( 3-methyl-2- benzothiazal)hydrazine Gp-diphenylaminocinnamic acid 1-1 4,4',4"-tris( methoxyphenyl )amine Jtri-p-tolylamine Certain of the compositions also contain a spectral dyesensitizer, as indicated in Table IV below.

TABLE 1V Element Concv No. Compound (g) Speed (Negtoe) 14 None (control)34 [5 E 0.02 98 [6 F 0.02 66 17 G 002 18 H 0.02 63 19 .l 0.02 20 .l IV0.02 0.02 21 J V 0.02 +0.02 225 It is seen that addition of chemicalsensitizers as indicated herein markedly increases theelectrophotographic response of the compositions and elements containingthem.

EXAMPLE 5 Coating compositions are made according to the controlcomposition of Example 3 but with varying concentrations of TNF. Theconcentration of TNF content is varied between about 12.5 percent andabout 50 percent, based on the weight of the total solids. Thecompositions are coated to form electrophotographic elements in themanner hereinbefore indicated, and the elements thus formed are testedelectrophotographically with both positive and negative polarities ofinitial charging. The toe and shoulder speeds, as hereinbefore defined,of these elements are given in Table V.

TABLE V Weight Speed Percent Positive Negative TN F Shoulder ToeShoulder Toe The speed figures given in Table V show that aconcentration of as high as 50 percent of the weight of the total solidsin the coating composition gives a decided improvement inelectrophotographic response. This is seen to be true even though theconcentration passes through an optimum for both positive and negativecharging.

The invention has been described in detail with particular reference topreferred embodiments thereof, but it will be understood that variationsand modifications can be effected within the spirit and scope of theinvention.

We claim:

1. An electrophotographic element comprising a conductive support havingcoated thereon a photoconductive composition comprising a complex formedby a resin selected from the group consisting ofdibenzofuran-formaldehyde resins and dibenzothiophene-formaldehyderesins and a Lewis acid selected from the group consisting of2,4,7-trinitro-9- fluorenone, 2,4,5,7-tetranitro-9fluorenone,2,6-dichloro-pbenzoquinone, 2,5-dinitro9-fluorenone, l,5-dichloro-2,4-dinitrobenzene, 2,5-dichloro-p-benzoquinone, tetrachloroquinone,2-chloro-3,S-dinitropyridine, 2,4,5,7,9- pentanitroindene[2.l]-fluoren-l l. l2-dione, 2, S-diphenyl-pbenzoquinone,2,3-dichloro-l,4-naphthoquinone, 9-dicyanomethylene-2,4,7-trinitrofluorene.

2. The element of claim 1 wherein said photoconductive compositioncontains a sensitizer for said photoconductive complex.

3. The element of claim 1 wherein the photoconductive complex is formedfrom dibenzofuran-formaldehyde resin and 2,4.7-trinitrofiuorenone.

4. The element of claim 1 wherein the photoconductive complex if formedfrom dibenzothiophene-formaldehyde resin and 2,4,7-trinitrofluorenone.

5. The element of claim 1 wherein said photoconductive compositioncontains a film-forming polymeric binder for said complex.

6. An electrophotographic element comprising a conductive support havingcoated thereon a photoconductive composition comprising a. from about0.01 to about percent by weight based on said photoconductivecomposition of a complex formed by a resin selected from the groupconsisting of dibenzofuran-formaldehyde resins anddibenzothiophene-formaldehyde resins and a compound selected from thegroup consisting of 2,4,7-trinitro-9- fluorenone,2,4,5,7-tetranitro-9-fluorenone, 2,6- dichloro-p-benzoquinone,2,5-dinitro-9-fluorenone, 1,5- dichloro-2,4-dinitrobenzene,2,S-dichloro-p-benzoquinone, tetrachloro-quinone,2-chloro-3,S-dinitro-pyridine, 2,4,5.7,9-pentanitr0-indene[2, lalfluoren-l l, lZ-dione. 2,5-diphenyl-p-benzoquinone, 2,3-dichlorol ,4-naphthoquinone, 9-dicyanomethylene-2,4,7- trinitrofluorene,

b. a film-forming polymeric binder for said complex and c. from about0.005 percent to about 5 percent by weight based on said photoconductivecomposition of a sensitizer for said photoconductive composition.

7. In an electrophotographic process wherein an electrostatic chargepattern is formed on an electrophotographic element, the improvementcharacterized in that said electrophotographic element has aphotoconductive layer comprising a complex of a Lewis acid and a resinselected from the group consisting of dibenzofuran-formaldehyde resinsand dibenzothiophene-formaldehyde resins.

8. The process of claim 7 wherein the photoconductive layer comprises acomplex formed between dibenzofuran-formaldehyde resin and 2,4,7-trinitrofluqrenone. I

9. The process of claim 7 wherein the photoconductive layer comprises acomplex formed between dibenzothiophene-formaldehyde resin and 2,4,7-trinitrofluorenone.

10. An electrophotographic element comprising a conductive supporthaving coated thereon a photoconductive composition comprising.

a. from about 0.01 to about 90 percent by weight based on saidphotoconductive composition of a complex formed by a resin selected fromthe group consisting of dibenzofuran-formaldehyde resins anddibenzothiophene-formaldehyde resins and 2, 4, 7- trinitro-9-fluorenone,

b. a film-forming polymeric binder for said complex, and

c. from about 0.005 percent to about 5 percent by weight based on saidphotoconductive composition of a sensitizer for said photoconductivecomposition.

2. The element of claim 1 wherein said photoconductive compositioncontains a sensitizer for said photoconductive complex.
 3. The elementof claim 1 wherein the photoconductive complex is formed fromdibenzofuran-formaldehyde resin and 2,4,7-trinitrofluorenone.
 4. Theelement of claim 1 wherein the photoconductive complex if formed fromdibenzothiophene-formaldehyde resin and 2,4,7-trinitrofluorenone.
 5. Theelement of claim 1 wherein said photoconductive composition contains afilm-forming polymeric binder for said complex.
 6. Anelectrophotographic element comprising a conductive support havingcoated thereon a photoconductive composition comprising a. from about0.01 to about 90 percent by weight based on said photoconductivecomposition of a complex formed by a resin selected from the groupconsisting of dibenzofuran-formaldehyde resins anddibenzothiophene-formaldehyde resins and a compound selected from thegroup consisting of 2,4,7-trinitro-9-fluorenone,2,4,5,7-tetranitro-9-fluorenone, 2,6-dichloro-p-benzoquinone,2,5-dinitro-9-fluorenone, 1,5-dichloro-2,4-dinitrobenzene,2,5-dichloro-p-benzoquinone, tetrachloro-quinone,2-chloro-3,5-dinitro-pyridine,2,4,5,7,9-pentanitro-indene(2,1-a)fluoren-11,12-dione,2,5-diphenyl-p-benzoquinone, 2,3-dichloro-1,4-naphthoquinone,9-dicyanomethylene-2,4,7-trinitrofluorene, b. a film-forming polymericbinder for said complex and c. from about 0.005 percent to about 5percent by weight based on said photoconductive composition of asensitizer for said photoconductive composition.
 7. In anelectrophotographic process wherein an electrostatic charge pattern isformed on an electrophotographic element, the improvement characterizedin that said electrophotographic element has a photoconductive layercomprising a complex of a Lewis acid and a resin selected from the groupconsisting of dibenzofuran-formaldehyde resins anddibenzothiophene-formaldehyde resins.
 8. The process of claim 7 whereinthe photoconductive layer comprises a complex formed betweendibenzofuran-formaldehyde resin and 2,4,7-trinitrofluorenone.
 9. Theprocess of claim 7 wherein the photoconductive layer comprises a complexformed between dibenzothiophene-formaldehyde resin and2,4,7-trinitrofluorenone.
 10. An electrophotographic element comprisinga conductive support having coated thereon a photoconductive compositioncomprising. a. from about 0.01 to about 90 percent by weight based onsaid photoconductive composition of a complex formed by a resin selectedfrom the group consisting of dibenzofuran-formaldehyde resins anddibenzothiophene-formaldehyde resins and 2, 4, 7-trinitro-9-fluorenone,b. a film-forming polymeric binder for said complex, and c. from about0.005 percent to about 5 percent by weight based on said photoconductivecomposition of a sensitizer for said photoconductive composition.